What does wartime blocking mean?

"Wartime blocking" is generally a sign placed in a city's basement or underground passage. It means that it can be used normally during peacetime. In the event of war, certain walls will be blocked. Forming an air-raid shelter can also prevent the invasion of smoke, poisonous gas, etc., and is used for emergency purposes. The picture shows the "wartime blockade" sign at the exit of the Xinjiekou underground passage taken by reporters in the morning. Photo by Fan Xiaolin